Some control valves are open-center valves, which utilize fixed displacement pumps to induce a constant flow in a hydraulic system. The constant flow allows quick operation of devices, such as actuators, to be operated quickly at full speed. Any excess flow induced by the pump is routed back to a tank to be filtered and re-circulated by the pump. Generally multiple open-center valves are arranged together in a configuration called a valve stack and each open-center valve is manually controlled by an operator.
Some other control valves are closed-center valves, which utilize variable displacement pumps to minimize energy loss from a constantly running fixed displacement pump. The variable displacement pumps provide pressure to each closed-center valve without a need to route fluid to tank because the flow is minimized when the corresponding valve is not actuated. Similar to open-center valves, generally multiple closed-center valves are arranged together in a valve stack and controlled by an operator, but the valves are connected in a parallel arrangement to provide equal pressure for all valves.
Some closed-center valves utilize load sensing, which allows the variable displacement pump to reduce or increase fluid flow and pressure to match load requirements of the system.